Need to render a VERY HQ 2.4 meter tall image
For print a 2.4 meter billboard you are absolutely don't need it to be in "HQ". Check this answers here.
You don't have to render all in once - you can render layers and then compose them. Explained here
I can presume that if you rendering a work for this kind of printing, you will have to make some post-production work - color correction, text layout etc. So the right workflow is to render the elements you need in blender and than combine them in your preferred editor, trying to make minimum layout work in blender.